ADT7421ARMZ-REEL [ONSEMI]
Precision Temperature Sensor with Beta Compensation (for <45 nm Geometries); 高精度温度传感器与beta补偿（对于\u003c 45纳米几何）
| 型号： | ADT7421ARMZ-REEL |
| 厂家： | 
ONSEMI |
| 描述： | Precision Temperature Sensor with Beta Compensation (for <45 nm Geometries) |
| 文件： | 总20页 (文件大小：246K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
ADT7421
Precision Temperature
Sensor with Beta
Compensation
(for <45 nm Geometries)
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The ADT7421 is a dual−channel digital thermometer and
under/overtemperature alarm, intended for use in PCs and thermal
management systems. It is pin− and register−compatible with the
ADM1032, ADT7461 and ADT7461A.
MARKING
DIAGRAMS
8
The ADT7421 includes Beta Cancellation Technology. This enables
accurate measurement of temperature from very small geometry
(45 nm) processes. Significant variations in the Beta factor can be
observed when different currents are applied to transistors embedded
in small geometry CPU’s. This leads to large temperature errors. The
ADT7421 automatically cancels the effects of error induced by beta
variations.
T7421
ALYWRG
G
SOIC−8
CASE 751
1
1
A
L
Y
W
R
G
= Assembly Location
= Wafer Lot
= Year
= Work Week
= SMBus Address
= Pb−Free Package
Features
• On−Chip and Remote Temperature Sensor
• 0.25°C Resolution on Remote Channel
• 1°C Resolution on Local Channel
• Automatically Cancels the Effect of Beta Variation in Thermal
Transistors on Small Geometry CPU’s
8
L7x
AYWG
G
MSOP−8
CASE 846AB
1
1
L7x = Refer to Ordering Table
A
Y
W
G
= Assembly Location
= Year
= Work Week
• Automatically Cancels Up to 50 W (Typical) of Resistance in Series
with Remote Transistor
• Extended, Switchable Temperature Measurement Range
= Pb−Free Package
0°C to +125°C (default) or −40°C to +125°C
(Note: Microdot may be in either location)
• Pin− and Register−Compatible with ADM1032, ADT7461,
ADT7461A, EMC1402, and aSC7525
PIN ASSIGNMENT
• 2−Wire SMBus Serial Interface with SMBus Alert Support
• Programmable Over/Undertemperature Limits
• Offset Registers for System Calibration
• Up to Two Overtemperature Fail−Safe THERM Outputs
• Small 8−lead MSOP and SOP Packages
• These are Pb−Free Devices
SCLK
V
1
2
3
4
8
7
6
5
DD
D+
D–
SDATA
ALERT/THERM2
GND
THERM
(Top View)
Applications
• Desktop and Notebook Computers
• Industrial Controllers
• Smart Batteries
ORDERING INFORMATION
See detailed ordering and shipping information in the package
dimensions section on page 18 of this data sheet.
• Automotive
• Embedded Systems
• Burn−In Applications
• Instrumentation
© Semiconductor Components Industries, LLC, 2010
1
Publication Order Number:
January, 2010 − Rev. 5
ADT7421/D
ADT7421
Another feature of the ADT7421 is Series Resistance
The ADT7421 has a configurable ALERT output and an
extended, switchable temperature measurement range.
The ADT7421 communicates over a 2−wire serial
interface, compatible with system management bus
(SMBus) standards. The default SMBus address of the
ADT7421 is 0x4C. An ADT7421−2 is available with a
SMBus address of 0x4D. This is useful if more than one
ADT7421 is used on the same SMBus.
An ALERT output signals when the on−chip or remote
temperature is out of range. The THERM output is a
comparator output that allows on/off control of a cooling
fan. The ALERT output can be reconfigured as a second
THERM output, if required.
Cancellation, where up to 50 W (typical) of resistance in
series with the temperature monitoring transistor can be
automatically cancelled from the temperature result,
allowing noise filtering.
The combination of Beta cancellation and series
resistance cancellation provides critical high accuracy
temperature sensing on 45 nm CPU’s and GPU’s.
The ADT7421 can measure the temperature of a remote
thermal transistor accurate to 1°C and the ambient
temperature accurate to 1°C. The temperature measurement
range defaults to 0°C to +125°C, compatible with the
ADM1032, but it can be switched to a wider measurement
range of −40°C to +125°C.
ADDRESS POINTER
REGISTER
ON−CHIP
TEMPERATURE
SENSOR
CONVERSION RATE
REGISTER
LOCAL TEMPERATURE
LOW−LIMIT REGISTER
LOCAL TEMPERATURE
VALUE REGISTER
LOCAL TEMPERATURE
HIGH−LIMIT REGISTER
REMOTE TEMPERATURE
2
3
D+
D–
A−TO−D
CONVERTER
LOW−LIMIT REGISTER
ANALOG
MUX
REMOTE TEMPERATURE
HIGH−LIMIT REGISTER
BUSY RUN/STANDBY
REMOTE TEMPERATURE
VALUE REGISTER
LOCAL THERM LIMIT
REGISTERS
EXTERNAL THERM LIMIT
REGISTERS
REMOTE OFFSET
REGISTER
CONFIGURATION
REGISTERS
EXTERNAL DIODE OPEN−CIRCUIT
INTERRUPT
MASKING
6
ALERT/THERM2
STATUS REGISTER
4
THERM
ADT7421
SMBus INTERFACE
1
5
7
8
V
GND
SDATA
SCLK
DD
Figure 1. Block Diagram
PIN ASSIGNMENT
Pin No.
Mnemonic
Description
1
2
3
4
V
Positive Supply, 3.0 V to 3.6 V.
DD
D+
D−
Positive Connection to Remote Temperature Sensor. (Anode)
Negative Connection to Remote Temperature Sensor. (Cathode)
THERM
Open-Drain Output. Can be used to turn a fan on/off or throttle a CPU clock in the event of an
overtemperature condition. Requires pullup resistor.
5
6
GND
Supply Ground Connection.
ALERT / THERM2
Open-Drain Logic Output Used as Interrupt or SMBus ALERT. This can also be configured as a
second THERM output. Requires pullup resistor.
7
8
SDATA
SCLK
Logic Input/Output, SMBus Serial Data. Open-Drain Output. Requires pullup resistor.
Logic Input, SMBus Serial Clock. Requires pullup resistor.
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ADT7421
MAXIMUM RATINGS
Parameter
Rating
Positive Supply Voltage (V ) to GND
−0.3 V, +3.6 V
DD
D+
−0.3 V to V + 0.3 V
DD
D− to GND
−0.3 V to +0.6 V
SCL, SDA, ALERT, THERM
Input Current, SDA, THERM2
Input Current, D−
−0.3 V to V +0.3 V
DD
−1 mA, +50 mA
−1 mA
ESD Rating, All Pins (Human Body Model)
ESD Rating, All Pins (Machine Model)
1500 V
100 V
Maximum Junction Temperature (T Max)
150°C
J
Storage Temperature Range
−65°C to +150°C
Stresses exceeding Maximum Ratings may damage the device. Maximum Ratings are stress ratings only. Functional operation above the
Recommended Operating Conditions is not implied. Extended exposure to stresses above the Recommended Operating Conditions may affect
device reliability.
NOTE: This device is ESD sensitive. Use standard ESD precautions when handling.
THERMAL RESISTANCE
Package Type
q
JA
q
JC
Unit
8−Lead MSOP
142
43.74
°C/W
SMBus Timing Specifications
Parameter (Note 1)
Limit at T
and T
Unit
kHz max
ms min
ms min
ns max
ns max
ns min
ns min
ns min
ns min
ms min
Description
MIN
MAX
f
400
SCLK
t
1.3
0.6
Clock low period, between 10% points
Clock high period, between 90% points
Clock/data rise time
LOW
HIGH
t
t
R
300
300
600
600
100
600
1.3
t
F
Clock/data fall time
t
Start condition setup time
SU; STA
t
t
(Note 2)
(Note 3)
(Note 4)
Start condition hold time
HD; STA
SU; DAT
SU; STO
Data setup time
t
Stop condition setup time
t
Bus free time between stop and start conditions
BUF
1. Guaranteed by design, but not production tested.
2. Time from 10% of SDATA to 90% of SCLK.
3. Time for 10% or 90% of SDATA to 10% of SCLK.
4. Time for 90% of SCLK to 10% of SDATA.
tR
tF
tHD;STA
tLOW
SCLK
tHIGH
tSU;DAT
tSU;STA
tHD;STA
tHD;DAT
tSU;STO
SDATA
tBUF
STOP START
START
STOP
Figure 2. Serial Bus Timing
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ADT7421
ELECTRICAL CHARACTERISTICS (T = −40°C to +125°C, V = 3.0 V to 3.6 V, unless otherwise noted)
A
DD
Parameter
Power Supply
Supply Voltage, V
Conditions
Min
Typ
Max
Unit
3.0
3.30
3.0
3.6
4.0
V
DD
Average Operating Supply Current, I
0.0625 Conversions/Sec Rate (Note 1)
−40°C ≤ T ≤ +85°C
mA
mA
DD
Standby Mode Supply Current
10
20
A
A
−40°C ≤ T ≤ +125°C
Power-On-Reset Threshold
Temperature−To−Digital Converter
Local Sensor Accuracy
1.0
2.8
V
0°C ≤ T ≤ +125°C
2.75
°C
°C
°C
A
Resolution
1.0
Remote Transistor Sensor Accuracy
+40°C ≤ T ≤ +85°C, +60°C ≤ T ≤ +110°C (Note 2)
2.5
3.25
1.75
A
D
D
+25°C ≤ T ≤ +85°C, +60°C ≤ T ≤ +110°C (Note 2)
A
T
A
= +40°C, +60°C ≤ T ≤ +110°C (Note 2)
D
Resolution
0.25
°C
mA
Remote Sensor Source Current
The range of source currents provided to the external
thermal transistor for temperature measurements.
10 to
360
Conversion Time
From stop bit to conversion complete, one-shot mode
with averaging switched on.
184
ms
One-shot mode with averaging off.
20
50
Maximum Series Resistance Cancelled Resistance split evenly on both the D+ and D– inputs.
W
Open−Drain Digital Outputs (THERM, ALERT/THERM2, SDA)
Output Low Voltage, V
I
= −6.0 mA
0.2
1.0
V
OL
OUT
High Level Output Leakage Current,
V
= V
0.1
mA
OUT
DD
I
OH
SMBus Interface (Note 3)
Logic Input High Voltage, V
SCL, SDA
3.0 V ≤ V ≤ 3.6 V
2.1
V
V
IH
DD
Logic Input Low Voltage, V
SCL, SDA
3.0 V ≤ V ≤ 3.6 V
0.8
IL
DD
Hysteresis
500
mV
V
SDA Output Low Voltage, V
I
= −6.0 mA
0.4
OL
OUT
Logic Input Current, I , I
−1.0
+1.0
mA
pF
IH IL
SMBus Input Capacitance,
SCLK, SDATA
10
25
SMBus Clock Frequency
SMBus Timeout (Note 4)
400
300
1.0
kHz
ms
ms
User programmable
SCLK Falling Edge to SDATA Valid
Time
Master clocking in data
1. See Table 4 for information on other conversion rates.
2. Guaranteed by characterization, but not production tested.
3. See SMBus Timing Specifications section for more information.
4. Disabled by default. Detailed procedures to enable it are in the Serial Bus Interface section of this datasheet.
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ADT7421
TYPICAL CHARACTERISTICS
1.00
0.75
0.50
0.25
0
80
60
40
D+ to GND
20
0
−0.25
−0.50
−20
−40
D+ to Vcc
−0.75
−1.00
−60
−80
1
10
100
1000
10,000
0
10 20 30 40 50 60 70 80 90 100
CAPACITANCE (pF)
LEAKAGE RESISTANCE (MW)
Figure 3. Temperature Error vs. D+, D−
Figure 4. Temperature Error vs. PCB Leakage
Resistance
Capacitance
5
4
5
4
3
3
2
2
1
1
0
0
−1
−2
−3
−1
−2
−3
−4
−5
−50 −25
−4
−5
−50 −25
0
25
50
75
100
125
150
0
25
50
75
100 125 150
T & T OIL BATH TEMPERATURE (°C)
T & T OIL BATH TEMPERATURE (°C)
A
D
A
D
Figure 5. Local Temperature Error vs.
Temperature
Figure 6. Remote Temperature Error vs.
Temperature
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
6
4
2
250 mV
100 mV
0
−2
−4
−6
0.1
0
0
20
40
60
80
100
120
0
1E+08 2E+08 3E+08 4E+08 5E+08 6E+08
NOISE FREQUENCY
RS (W)
Figure 7. Temperature Error vs. Series
Figure 8. External Temp Error vs. Power
Supply Noise
Resistance on D+, D−
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ADT7421
Theory of Operation
accurate result, without the need for user characterization of
this resistance. The ADT7421 is designed to automatically
cancel typically up to 50 W of resistance. By using an
advanced temperature measurement method, this process is
transparent to the user.
The ADT7421 is a local and remote temperature sensor
and over/undertemperature alarm, with the added ability to
automatically cancel the effect of beta variations in
embedded thermal transistors in small geometry CPU’s.
When the ADT7421 is operating normally, the on−board
ADC operates in a free running mode. The analog input
multiplexer alternately selects either the on−chip
temperature sensor to measure its local temperature or the
remote temperature sensor. The ADC digitizes these signals
and the results are stored in the local and remote temperature
value registers.
The local and remote measurement results are compared
with the corresponding high, low, and THERM temperature
limits, stored in eight on−chip registers. Out−of−limit
comparisons generate flags that are stored in the status
register. A result that exceeds the high temperature limit or
the low temperature limit causes the ALERT output to
assert. The ALERT output also asserts if an external
transistor fault is detected. Exceeding the THERM
temperature limits causes the THERM output to assert low.
The ALERT output can be reprogrammed as a second
THERM output.
Temperature Measurement Method
A simple method of measuring temperature is to exploit
the negative temperature coefficient of a transistor,
measuring the base emitter voltage (V ) of a transistor
BE
operated at constant current. However, this technique
requires calibration to null the effect of the absolute value of
V , which varies from device to device.
BE
The technique used in the ADT7421 measures the change
in V when the device operates at three different currents.
BE
Previous devices used only two operating currents, but it is
the use of a third current that allows automatic cancellation
of resistances in series with the external temperature sensor.
Figure 9 shows the input signal conditioning used to
measure the output of an external temperature sensor. This
figure shows the external sensor as a substrate transistor, but
it can equally be a discrete transistor. If a discrete transistor
is used, the collector is not grounded but is linked to the base.
To prevent ground noise interfering with the measurement,
the more negative terminal of the sensor is not referenced to
ground, but is biased above ground by an internal transistor
at the D− input. C1 may be added as a noise filter (a
recommended maximum value of 2200 pF).
The limit registers are programmed and the device
controlled and configured via the serial SMBus. The
contents of any register are also read back via the SMBus.
Control and configuration functions consist of switching
the device between normal operation and standby mode,
selecting the temperature measurement range, masking or
enabling the ALERT output, switching Pin 6 between
ALERT and THERM2, and selecting the conversion rate.
To measure DV , the operating current through the
BE
sensor is switched among three related currents. As shown
in Figure 9, N1 × I and N2 × I are different multiples of the
current, I. The currents through the temperature transistor
Beta Variation Cancellation
are switched between I and N1 × I, giving V ; and then
BE1
The ADT7421 includes a new temperature sensing
method which cancels out the effect of varying Beta factors
being observed when different currents are applied to the
embedded thermal transistor in small geometry processes.
This method also ensure consistent and accurate
temperature measurements between CPU’s.
between I and N2 × I, giving DV . The temperature is then
BE2
calculated using the two DV measurements. This method
BE
also cancels the effect of any series resistance on the
temperature measurement.
The resulting DV waveforms are passed through a
BE
65 kHz low−pass filter to remove noise and then to a
chopper−stabilized amplifier. This amplifies and rectifies
Series Resistance Cancellation
the waveform to produce a dc voltage proportional to DV
.
BE
Parasitic resistance to the D+ and D− inputs to the
ADT7421, seen in series with the remote transistor, is
caused by a variety of factors, including PCB track
resistance and track length. This series resistance appears as
a temperature offset in the remote sensor’s temperature
measurement. This error typically causes a 0.5°C offset per
ohm of parasitic resistance in series with the remote
transistor.
The ADC digitizes this voltage producing a temperature
measurement. To reduce the effects of noise, digital filtering
is performed by averaging the results of 16 measurement
cycles for low conversion rates. At rates of 10, 20, and
36 conversions per second, no digital averaging occurs.
Signal conditioning and measurement of the internal
temperature sensor are performed in the same manner.
The ADT7421 automatically cancels the effect of this
series resistance on the temperature reading, giving a more
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ADT7421
V
DD
I
BIAS
I
N2 y I
N1 y I
V
OUT+
D+
TO ADC
C1
D–
REMOTE
SENSING
TRANSISTOR
fC = 65kHz
V
OUT–
BIAS
DIODE
NOTE:
CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS. C1 = 1000pF MAX.
Figure 9. Input Signal Conditioning
Temperature Measurement Results
can have values from −40°C to +125°C. Most temperature
sensing transistors have a maximum temperature range of
−55°C to +150°C. Above +150°C, they may lose their
semiconductor characteristics and approximate conductors
instead.
The results of the local and remote temperature
measurements are stored in the local and remote temperature
value registers and compared with limits programmed into
the local and remote high and low limit registers.
The local temperature value is in Register 0x00 and has a
resolution of 1°C. The external temperature value is stored
in two registers, with the upper byte in Register 0x01 and the
lower byte in Register 0x10. Only the two MSBs in the
external temperature low byte are used giving the external
temperature measurement a resolution of 0.25°C. The table
lists the data format for the external temperature low byte.
It should be noted that although both local and remote
temperature measurements can be made while the part is in
extended temperature mode, the ADT7421 itself should not
be exposed to temperatures greater than those specified in
the Absolute Maximum Ratings Table. Further, the device
is only guaranteed to operate as specified at ambient
temperatures from −40°C to +125°C.
Table 1. Extended Temperature Resolution
(Remote Temperature Low Byte)
Temperature Data Format
The ADT7421 has two temperature data formats. When
the temperature measurement range is from 0°C to 125°C
(default), the temperature data format for both internal and
external temperature results is binary. When the
measurement range is in extended mode, an offset binary
data format is used for both internal and external results.
Temperature values are offset by 64°C in the offset binary
data format. Examples of temperatures in both data formats
are shown in the following table.
Extended Resolution
0.00°C
Remote Temperature Low Byte
0 000 0000
0.25°C
0 100 0000
0.50°C
1 000 0000
0.75°C
1 100 0000
When reading the full external temperature value, read the
LSB first. This causes the MSB to be locked (that is, the
ADC does not write to it) until it is read. This feature ensures
that the results read back from the two registers come from
the same measurement.
Table 2. Temperature Data Format
(Temperature High Byte)
Temperature
−40°C
Binary
Offset Binary
0 001 1000
0 100 0000
0 100 0001
0 100 1010
0 101 1001
0 111 0010
1 000 1011
1 010 0100
1 011 1101
Temperature Measurement Range
0 000 0000
0 000 0000
0 000 0001
0 000 1010
0 001 1001
0 011 0010
0 100 1011
0 110 0100
0 111 1101
The temperature measurement range for both internal and
external measurements is, by default, 0°C to +125°C.
However, the ADT7421 can be operated using an extended
temperature range. The extended measurement range is
−40°C to +125°C.
The extended temperature range is selected by setting
Bit 2 of the configuration register to 1. The temperature
range is 0°C to 125°C when Bit 2 equals 0. A valid result is
available in the next measurement cycle after changing the
temperature range.
0°C
+1°C
+10°C
+25°C
+50°C
+75°C
+100°C
+125°C
1. Offset binary scale temperature values are offset by 64°C.
2. Binary scale temperature measurement returns 0°C for all
temperatures < 0°C.
3. Binary scale temperature measurement returns 125°C for all
temperatures > 125°C.
In extended temperature mode, the upper and lower
temperature that can be measured by the ADT7421 is limited
by the remote transistor selection. The temperature registers
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ADT7421
The user can switch between measurement ranges at any
1, the device is in standby mode and the ADC does not
convert. The SMBus does, however, remain active in standby
mode; therefore, values can be read from or written to the
ADT7421 via the SMBus. The ALERT and THERM outputs
are also active in standby mode. Changes made to the
registers in standby mode that affect the THERM or ALERT
outputs cause these signals to be updated.
Bit 4 switches beta cancellation on and off. With Bit 4 set
to zero beta cancellation is set on. If using a discrete
transistor as the sensing element, Beta Cancellation should
be switched off by setting Bit 4 to 1.
Bit 5 determines the configuration of Pin 6 on the
ADT7421. If Bit 5 is 0 (default), then Pin 6 is configured as
an ALERT output. If Bit 5 is 1, then Pin 6 is configured as
a THERM2 output. Bit 7, the ALERT mask bit, is only active
when Pin 6 is configured as an ALERT output. If Pin 6 is set
up as a THERM2 output, then Bit 7 has no effect.
Bit 2 sets the temperature measurement range. If Bit 2 is
0 (default value), the temperature measurement range is set
between 0°C to +125°C. Setting Bit 2 to 1 sets the
measurement range to the extended temperature range
(−40°C to +125°C).
time. Switching the range likewise switches the data format.
The next temperature result following the switching is
reported back to the register in the new format. However, the
contents of the limit registers do not change. It is up to the
user to ensure that when the data format changes, the limit
registers are reprogrammed as necessary. More information
on this is found in the Limit Registers section.
ADT7421 Registers
The ADT7421 contains 22, 8−bit registers in total. These
registers store the results of remote and local temperature
measurements, high and low temperature limits, and
configure and control the device. See the section through the
Consecutive ALERT Register section of this data sheet for
more information on the ADT7421 registers. Additional
details are shown in Table 3 through Table 7. The entire
register map is available in Table 8.
Address Pointer Register
The address pointer register itself does not have, nor does
it require, an address because the first byte of every write
operation is automatically written to this register. The data
in this first byte always contains the address of another
register on the ADT7421 that is stored in the address pointer
register. It is to this register address that the second byte of
a write operation is written, or to which a subsequent read
operation is performed.
Table 3. Configuration Register Bit Assignments
Power−On
Default
Bit
Name
MASK1
Function
7
0 = ALERT Enabled
1 = ALERT Masked
0
The power−on default value of the address pointer register
is 0x00. Therefore, if a read operation is performed
immediately after power−on, without first writing to the
address pointer, the value of the local temperature is returned
because its register address is 0x00.
6
RUN/STOP
0 = Run
1 = Standby
0
5
4
ALERT/
THERM2
0 = ALERT
1 = THERM2
0
0
Beta Enable
0 = Beta Compensation
On
1 = Beta Compensation
Off
Temperature Value Registers
The ADT7421 has three registers to store the results of
local and remote temperature measurements. These
registers can only be written to by the ADC and can be read
by the user over the SMBus. The local temperature value
register is at Address 0x00.
The external temperature value high byte register is at
Address 0x01, with the low byte register at Address 0x10.
The power−on default for all three registers is 0x00.
3
2
Reserved
Reserved
1
0
Temperature
Range Select
0 = 0°C to 125°C
1 = Extended Range
1
0
Reserved
Reserved
Reserved
Reserved
0
0
Conversion Rate Register
The conversion rate register is Address 0x04 at read and
Address 0x0A at write. The lowest four bits of this register
are used to program the conversion rate. For example, a
conversion rate of five conversions per second means that
beginning at 200 ms intervals, the device performs a
conversion on the internal and the external temperature
channels.
The conversion rate register can be written to and read
back over the SMBus. The higher four bits of this register are
unused and must be set to 0. The default value of this register
is 0x06, giving a rate of 4 conversions per second. Use of
slower conversion times greatly reduces the device power
consumption.
Configuration Register
The configuration register is Address 0x03 at read and
Address 0x09 at write. Its power−on default is 0x08. Only
five bits of the configuration register are used. Bit 0, Bit 1,
and Bit 3 are reserved; the user does not write to them.
Bit 7 of the configuration register masks the ALERT
output. If Bit 7 is 0, the ALERT output is enabled. This is the
power−on default. If Bit 7 is set to 1, the ALERT output is
disabled. This applies only if Pin 6 is configured as ALERT.
If Pin 6 is configured as THERM2, then the value of Bit 7
has no effect.
If Bit 6 is set to 0, which is power−on default, the device
is in operating mode with ADC converting. If Bit 6 is set to
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ADT7421
Table 4. Conversion Rate Register Codes
When Bit 7 of the status register is high, it indicates that the
ADC is busy converting. The other bits in this register flag the
out−of−limit temperature measurements (Bit 6 to Bit 3, and
Bit 1 to Bit 0) and the remote sensor open circuit (Bit 2).
If Pin 6 is configured as an ALERT output, the following
applies: If the local temperature measurement exceeds its
limits, Bit 6 (high limit) or Bit 5 (low limit) of the status
register asserts to flag this condition. If the remote
temperature measurement exceeds its limits, then Bit 4 (high
limit) or Bit 3 (low limit) asserts. Bit 2 asserts to flag an open
circuit condition on the remote sensor. These five flags are
NOR’ed together, so if any of them is high, the ALERT
interrupt latch is set and the ALERT output goes low.
Reading the status register clears the five flags, Bit 6 to
Bit 2, provided the error conditions causing the flags to be
set have gone away. A flag bit can be reset only if the
corresponding value register contains an in−limit
measurement or if the sensor is good.
Code
0x00
Conversion/Second
Time (Secs)
0.0625
16
8
0x01
0.125
0x02
0.25
4
0x03
0.5
2
0x04
1
1
0x05
2
500 m
250 m
200 m
100 m
50 m
27 m
0x06
4 (Default)
0x07
5
10
0x08
0x09
20
0x0A
36
0x0B to 0xFF
Reserved
Limit Registers
The ADT7421 has eight limit registers: high, low, and
THERM temperature limits for both local and remote
temperature measurements. The remote temperature high
and low limits span two registers each, to contain an upper
and lower byte for each limit. There is also a THERM
hysteresis register. All limit registers can be written to, and
read back over, the SMBus. See Table 8 for details of the
limit register addresses and their power−on default values.
When Pin 6 is configured as an ALERT output, the high
limit registers perform a > comparison, while the low limit
registers perform a ≤ comparison. For example, if the high
limit register is programmed with 80°C, then measuring 81°C
results in an out−of−limit condition, setting a flag in the status
register. If the low limit register is programmed with 0°C,
measuring 0°C or lower results in an out−of−limit condition.
Exceeding either the local or remote THERM limit asserts
THERM low. When Pin 6 is configured as THERM2,
exceeding either the local or remote high limit asserts
THERM2 low. A default hysteresis value of 10°C is provided
that applies to both THERM channels. This hysteresis value
can be reprogrammed to any value after powerup (Register
Address 0x21).
The ALERT interrupt latch is not reset by reading the
status register. It resets when the ALERT output has been
serviced by the master reading the device address, provided
the error condition has gone away and the status register flag
bits are reset.
When Flag 1 and/or Flag 0 are set, the THERM output
goes low to indicate that the temperature measurements are
outside the programmed limits. The THERM output does
not need to be reset, unlike the ALERT output. Once the
measurements are within the limits, the corresponding status
register bits are automatically reset and the THERM output
goes high. The user may add hysteresis by programming
Register 0x21. The THERM output is reset only when the
temperature falls to limit value minus the hysteresis value.
When Pin 6 is configured as THERM2, only the high
temperature limits are relevant. If Flag 6 and/or Flag 4 are
set, the THERM2 output goes low to indicate that the
temperature measurements are outside the programmed
limits. Flag 5 and Flag 3 have no effect on THERM2. The
behavior of THERM2 is otherwise the same as THERM.
Table 5. Status Register Bit Assignments
It is important to remember that the temperature limits data
format is the same as the temperature measurement data
format. Therefore, if the temperature measurement uses
default binary, then the temperature limits also use the binary
scale. If the temperature measurement scale is switched,
however, the temperature limits do not automatically switch.
The user must reprogram the limit registers to the desired
value in the correct data format. For example, if the remote
low limit is set at 10°C with the default binary scale, the limit
register value is 0000 1010b. If the scale is switched to offset
binary, the value in the low temperature limit register needs
to be reprogrammed to 0100 1010b.
Bit
7
Name
BUSY
Function
1 when ADC Converting
6
LHIGH*
LLOW*
RHIGH*
1 when Local High Temperature Limit Tripped
1 when Local Low Temperature Limit Tripped
5
4
1 when Remote High Temperature Limit
Tripped
3
RLOW*
1 when Remote Low Temperature Limit
Tripped
2
1
0
OPEN*
RTHRM
LTHRM
1 when Remote Sensor Open Circuit
1 when Remote THERM Limit Tripped
1 when Local THERM Limit Tripped
*These flags stay high until the status register is read or they are
reset by POR unless Pin 6 is configured as THERM2. Then,
only Bit 2 remains high until the status register is read or is reset
by POR.
Status Register
The status register is a read−only register at Address 0x02.
It contains status information for the ADT7421.
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ADT7421
Offset Register
One−Shot Register
Offset errors can be introduced into the remote
temperature measurement by clock noise or when the
thermal transistor is located away from the hot spot. To
achieve the specified accuracy on this channel, these offsets
must be removed.
The offset value is stored as a 10−bit, twos complement
value in Register 0x11 (high byte) and Register 0x12 (low
byte, left justified). Only the upper two bits of Register 0x12
are used. The MSB of Register 0x11 is the sign bit. The
minimum, programmable offset is −128°C, and the
maximum is +127.75°C. The value in the offset register is
added to, or subtracted from, the measured value of the
remote temperature.
The one−shot register is used to initiate a conversion and
comparison cycle when the ADT7421 is in standby mode,
after which the device returns to standby. Writing to the
one−shot register address (0x0F) causes the ADT7421 to
perform a conversion and comparison on both the internal
and the external temperature channels. This is not a data
register as such, and it is the write operation to Address 0x0F
that causes the one−shot conversion. The data written to this
address is irrelevant and is not stored.
Consecutive ALERT Register
The value written to this register determines how many
out−of−limit measurements must occur before an ALERT is
generated. The default value is that one out−of−limit
measurement generates an ALERT. The maximum value that
can be chosen is 4. The purpose of this register is to allow the
user to perform some filtering of the output. This is
particularly useful at the fastest three conversion rates, where
no averaging takes place. This register is at Address 0x22.
The offset register powers up with a default value of 0°C
and has no effect unless the user writes a different value to it.
Table 6. Sample Offset Register Codes
Offset Value
−128°C
−4°C
0x11
0x12
1000 0000
1111 1100
1111 1111
1111 1111
0000 0000
0000 0000
0000 0001
0000 0100
0111 1111
00 00 0000
00 00 0000
00 00 0000
11 00 0000
00 00 0000
01 00 0000
00 00 0000
00 00 0000
11 00 0000
Table 7. Consecutive ALERT Register 0x22
−1°C
Number of Out−of−Limit
Measurements Required
Bits
Value†
−0.25°C
0°C
<3−0>
000x
001x
011x
111x
1
2
3
4
+0.25°C
+1°C
+4°C
†x = Don’t care bit.
+127.75°C
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ADT7421
Table 8. List of Registers
Power−On Default
Read Address
(Hex)
Write Address
Name
(Hex)
Binary
Hex
Decimal
Not Applicable
Not Applicable
Address Pointer
Undefined
00
Not Applicable
Local Temperature Value
External Temperature Value High Byte
Status
0000 0000
0000 0000
Undefined
0000 1000
0000 1000
0101 0101
0000 0000
0101 0101
0000 0000
0x00
0x00
01
Not Applicable
02
Not Applicable
03
09
Configuration
0x08
0x06
0x55
0x00
0x55
0x00
04
0A
Conversion Rate
05
0B
Local Temperature High Limit
Local Temperature Low Limit
External Temperature High Limit High Byte
External Temperature Low Limit High Byte
One−Shot
85°C
0°C
06
0C
07
0D
85°C
0°C
08
0E
Not Applicable
0F
10
11
Not Applicable
External Temperature Value Low Byte
External Temperature Offset High Byte
External Temperature Offset Low Byte
External Temperature High Limit Low Byte
External Temperature Low Limit Low Byte
External THERM Limit
0000 0000
0000 0000
0000 0000
0000 0000
0000 0000
0101 0101
0101 0101
0000 1010
0000 0001
0010 0001
0100 0001
0x00
0x00
0x00
0x00
0x00
0x55
0x55
0x0A
0x01
0x21
0x41
11
12
13
14
19
20
21
22
3D
FE
FF
12
13
14
19
85°C
85°C
10°C
20
Local THERM Limit
21
THERM Hysteresis
22
Consecutive ALERT
Not Applicable
Not Applicable
Not Applicable
Device ID
Manufacturer ID
Die Revision Code
Serial Bus Interface
remains high. This indicates that an address/data stream
follows. All slave peripherals connected to the serial bus
respond to the start condition and shift in the next eight bits,
consisting of a 7−bit address (MSB first) plus an R/W bit,
which determines the direction of the data transfer, that is,
whether data is written to, or read from, the slave device. The
peripheral whose address corresponds to the transmitted
address responds by pulling the data line low during the low
period before the ninth clock pulse, known as the
acknowledge bit. All other devices on the bus remain idle
while the selected device waits for data to be read from or
written to it. If the R/W bit is a 0, the master writes to the
slave device. If the R/W bit is a 1, the master reads from the
slave device.
Control of the ADT7421 is carried out via the serial bus.
The ADT7421 is connected to this bus as a slave device,
under the control of a master device.
The ADT7421 has an SMBus timeout feature. When this
is enabled, the SMBus times out after typically 25 ms of no
activity. However, this feature is not enabled by default.
Bits 6 and
7
of the consecutive alert register
(Address = 0x22) should be set to enable it.
Addressing the Device
In general, every SMBus device has a 7−bit device
address, except for some devices that have extended 10−bit
addresses. When the master device sends a device address
over the bus, the slave device with that address responds.
The ADT7421is available with one device address, 0x4C
(1001 100b). The ADT7421−2 is also available.
The ADT7421−2 has an SMBus address of 0x4D (1001
101b). This is to allow two ADT7421 devices on the same
bus, or if the default address conflicts with an existing device
on the SMBus. The serial bus protocol operates as follows:
The master initiates a data transfer by establishing a start
condition, defined as a high−to−low transition on SDATA,
the serial data line, while SCLK, the serial clock line,
1. Data is sent over the serial bus in a sequence of
nine clock pulses, eight bits of data followed by an
acknowledge bit from the slave device. Transitions
on the data line must occur during the low period
of the clock signal and remain stable during the
high period, since a low−to−high transition when
the clock is high can be interpreted as a stop
signal. The number of data bytes that can be
transmitted over the serial bus in a single read or
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ADT7421
write operation is limited only by what the master
and slave devices can handle.
changed without starting a new operation. For the
ADT7421, write operations contain either one or two bytes,
while read operations contain one byte.
2. When all data bytes have been read or written,
stop conditions are established. In write mode, the
master pulls the data line high during the tenth
clock pulse to assert a stop condition. In read
mode, the master device overrides the
To write data to one of the device data registers, or to read
data from it, the address pointer register must be set so that
the correct data register is addressed. The first byte of a write
operation always contains a valid address that is stored in the
address pointer register. If data is to be written to the device,
the write operation contains a second data byte that is written
to the register selected by the address pointer register.
This procedure is illustrated in Figure 10. The device
address is sent over the bus followed by R/W set to 0. This
is followed by two data bytes. The first data byte is the
address of the internal data register to be written to, which
is stored in the address pointer register. The second data byte
is the data to be written to the internal data register.
acknowledge bit by pulling the data line high
during the low period before the ninth clock pulse.
This is known as no acknowledge. The master
takes the data line low during the low period
before the tenth clock pulse, then high during the
tenth clock pulse to assert a stop condition.
Any number of bytes of data are transferable over the
serial bus in one operation, but it is not possible to mix read
and write in one operation because the type of operation is
determined at the beginning and cannot subsequently be
1
9
1
9
SCLK
SDATA
A6
A5
A4
A3
A2
A1
A0
R/W
D7
D6
D5
D4
D3
D2
D1
D0
ACK. BY
ADT7421
ACK. BY
ADT7421
START BY
MASTER
FRAME 1
FRAME 2
SERIAL BUS ADDRESS BYTE
ADDRESS POINTER REGISTER BYTE
1
9
SCLK (CONTINUED)
SDATA (CONTINUED)
D7
D6
D5
D4
D3
D2
D1
D0
ACK. BY STOP BY
ADT7421 MASTER
FRAME 3
DATA BYTE
Figure 10. Writing a Register Address to the Address Pointer Register,
then Writing Data to the Selected Register
1
9
1
9
SCLK
SDATA
A6
A5
A4
A3
A2
A1
A0
R/W
D7
D6
D5
D4
D3
D2
D1
D0
ACK. BY
ADT7421
ACK. BY STOP BY
ADT7421 MASTER
START BY
MASTER
FRAME 2
FRAME 1
ADDRESS POINTER REGISTER BYTE
SERIAL BUS ADDRESS BYTE
Figure 11. Writing to the Address Pointer Register Only
1
9
1
9
SCLK
SDATA
A6
A5
A4
A3
A2
A1
A0
R/W
D7
D6
D5
D4
D3
D2
D1
D0
ACK. BY
ADT7421
ACK. BY STOP BY
ADT7421 MASTER
START BY
MASTER
FRAME 2
FRAME 1
ADDRESS POINTER REGISTER BYTE
SERIAL BUS ADDRESS BYTE
Figure 12. Reading from a Previously Selected Register
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ADT7421
When reading data from a register there are two
1. SMBALERT is pulled low.
possibilities.
2. Master initiates a read operation and sends the
alert response address (ARA = 0001 100). This is
a general call address that must not be used as a
specific device address.
3. The device whose ALERT output is low responds
to the alert response address and the master reads
its device address. As the device address is seven
bits, an LSB of 1 is added. The address of the
device is now known and it can be interrogated in
the usual way.
• If the address pointer register value of the ADT7421 is
unknown or not the desired value, it is first necessary to
set it to the correct value before data can be read from
the desired data register. This is done by writing to the
ADT7421 as before, but only the data byte containing
the register read address is sent, because data is not to
be written to the register (see Figure 11).
A read operation is then performed consisting of the
serial bus address, R/W bit set to 1, followed by the
data byte read from the data register (see Figure 12).
4. If more than one device’s ALERT output is low,
the one with the lowest device address takes
priority, in accordance with normal SMBus
arbitration.
• If the address pointer register is known to be at the
desired address, data can be read from the
corresponding data register without first writing to the
address pointer register and the bus transaction shown
in Figure 11 can be omitted.
Once the ADT7421 has responded to the alert response
address, it resets its ALERT output, provided that the error
condition that caused the ALERT no longer exists. If the
SMBALERT line remains low, the master sends the ARA
again, and so on until all devices whose ALERT outputs
were low have responded.
Notes
• It is possible to read a data byte from a data register
without first writing to the address pointer register.
However, if the address pointer register is already at the
correct value, it is not possible to write data to a register
without writing to the address pointer register because
the first data byte of a write is always written to the
address pointer register.
• Some of the registers have different addresses for read
and write operations. The write address of a register
must be written to the address pointer if data is to be
written to that register, but it may not be possible to
read data from that address. The read address of a
register must be written to the address pointer before
data can be read from that register.
Low Power Standby Mode
The ADT7421 can be put into low power standby mode
by setting Bit 6 of the configuration register. When Bit 6 is
low, the ADT7421 operates normally. When Bit 6 is high,
the ADC is inhibited, and any conversion in progress is
terminated without writing the result to the corresponding
value register. However, the SMBus is still enabled. Power
consumption in the standby mode is reduced to 10 mA.
When the device is in standby mode, it is possible to
initiate a one−shot conversion of both channels by writing to
the one−shot register (Address 0x0F), after which the device
returns to standby. It does not matter what is written to the
one−shot register, all data written to it is ignored. It is also
possible to write new values to the limit register while in
standby mode. If the values stored in the temperature value
registers are outside the new limits, an ALERT is generated,
even though the ADT7421 is still in standby.
ALERT Output
This is applicable when Pin 6 is configured as an ALERT
output. The ALERT output goes low whenever an
out−of−limit measurement is detected, or if the remote
temperature sensor is open circuit. It is an open−drain output
and requires a pullup resistor. Several ALERT outputs can
be wire−OR’ed together, so that the common line goes low
if one or more of the ALERT outputs goes low.
The ALERT output can be used as an interrupt signal to a
processor, or as an SMBALERT. Slave devices on the SMBus
cannot normally signal to the bus master that they want to
talk, but the SMBALERT function allows them to do so.
One or more ALERT outputs can be connected to a
common SMBALERT line that is connected to the master.
When the SMBALERT line is pulled low by one of the
devices, the following procedure occurs (see Figure 13):
Sensor Fault Detection
At its D+ input, the ADT7421 contains internal sensor
fault detection circuitry. This circuit can detect situations
where an external remote transistor is either not connected
or incorrectly connected to the ADT7421. A simple voltage
comparator trips if the voltage at D+ exceeds V − 1.0 V
DD
(typical), signifying an open circuit between D+ and D−. The
output of this comparator is checked when a conversion is
initiated. Bit 2 of the status register (open flag) is set if a fault
is detected. If the ALERT pin is enabled, setting this flag
causes ALERT to assert low.
MASTER
If the user does not wish to use an external sensor with the
ADT7421, tie the D+ and D− inputs together to prevent
continuous setting of the open flag.
RECEIVES
SMBALERT
ALERT RESPONSE
ADDRESS
DEVICE
NO
STOP
START
RD ACK
ADDRESS
ACK
MASTER SENDS
ARA AND READ
COMMAND
DEVICE SENDS
ITS ADDRESS
Figure 13. Use of SMBALERT
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ADT7421
TEMPERATURE
1005C
The ADT7421 Interrupt System
905C
805C
705C
605C
505C
405C
The ADT7421 has two interrupt outputs, ALERT and
THERM. Both have different functions and behavior.
ALERT is maskable and responds to violations of software
programmed temperature limits or an open−circuit fault on
the external transistor. THERM is intended as a fail−safe
interrupt output that cannot be masked.
If the external or local temperature exceeds the
programmed high temperature limits, or equals or exceeds the
low temperature limits, the ALERT output is asserted low. An
open−circuit fault on the external transistor also causes
ALERT to assert. ALERT is reset when serviced by a master
reading its device address, provided the error condition has
gone away and the status register has been reset.
The THERM output asserts low if the external or local
temperature exceeds the programmed THERM limits.
THERM temperature limits should normally be equal to or
greater than the high temperature limits. THERM is reset
automatically when the temperature falls back within the
THERM limit. The external and local limits are set by default
to 85°C. A hysteresis value can be programmed; in which
case, THERM resets when the temperature falls to the limit
value minus the hysteresis value. This applies to both local
and remote measurement channels. The power−on hysteresis
default value is 10°C, but this can be reprogrammed to any
value between 0°C and 31°C after powerup.
THERM LIMIT
THERM LIMIT−
HYSTERESIS
HIGH TEMP LIMIT
RESET BY MASTER
1
4
ALERT
THERM
2
3
Figure 14. Operation of the ALERT and THERM
Interrupts
• If the measured temperature exceeds the high
temperature limit, the ALERT output asserts low.
• If the temperature continues to increase and exceeds the
THERM limit, the THERM output asserts low. This can
be used to throttle the CPU clock or switch on a fan.
• The THERM output de−asserts (goes high) when the
temperature falls to THERM limit minus hysteresis. In
Figure 14, the default hysteresis value of 10°C is shown.
• The ALERT output de−asserts only when the
temperature has fallen below the high temperature
limit, and the master has read the device address and
cleared the status register.
The hysteresis loop on the THERM outputs is useful when
THERM is used, for example, as an on/off controller for a
fan. The user’s system can be set up so that when THERM
asserts, a fan is switched on to cool the system. When
THERM goes high again, the fan can be switched off.
Programming a hysteresis value protects from fan jitter,
where the temperature hovers around the THERM limit, and
the fan is constantly switched.
• Pin 6 on the ADT7421 can be configured as either an
ALERT output or as an additional THERM output.
• THERM2 asserts low when the temperature exceeds the
programmed local and/or remote high temperature
limits. It is reset in the same manner as THERM and is
not maskable.
• The programmed hysteresis value also applies to
THERM2.
Table 9. THERM Hysteresis
Figure 15 shows how THERM and THERM2 operate
together to implement two methods of cooling the system. In
this example, the THERM2 limits are set lower than the
THERM limits. The THERM2 output is used to turn on a
fan. If the temperature continues to rise and exceeds the
THERM limits, the THERM output provides additional
cooling by throttling the CPU.
THERM Hysteresis
Binary Representation
0 000 0000
0°C
1°C
0 000 0001
10°C
0 000 1010
Figure 14 shows how the THERM and ALERT outputs
operate. The ALERT output can be used as a SMBALERT
to signal to the host via the SMBus that the temperature has
risen. The user can use the THERM output to turn on a fan
to cool the system, if the temperature continues to increase.
This method ensures that there is a fail−safe mechanism to
cool the system, without the need for host intervention.
TEMPERATURE
905C
805C
THERM LIMIT
705C
605C
505C
THERM2 LIMIT
405C
305C
1
4
THERM2
THERM
3
2
Figure 15. Operation of the THERM and THERM2
Interrupts
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ADT7421
If a discrete transistor is used with the ADT7421, the best
accuracy is obtained by choosing devices according to the
following criteria:
• Base−emitter voltage greater than 0.25 V at 6 mA, at the
highest operating temperature
• Base−emitter voltage less than 0.95 V at 100 mA, at the
lowest operating temperature
• When the THERM2 limit is exceeded, the THERM2
signal asserts low.
• If the temperature continues to increase and exceeds the
THERM limit, the THERM output asserts low.
• The THERM output de−asserts (goes high) when the
temperature falls to THERM limit minus hysteresis. In
Figure 15, there is no hysteresis value shown.
• Base resistance less than 100 W
• As the system cools further, and the temperature falls
below the THERM2 limit, the THERM2 signal resets.
Again, no hysteresis value is shown for THERM2.
Both the external and internal temperature measurements
cause THERM and THERM2 to operate as described.
• Small variation in h (50 to 150) that indicates tight
FE
control of V characteristics
BE
Transistors, such as the 2N3904, 2N3906, or equivalents
in SOT−23 packages are suitable devices to use.
Thermal Inertia and Self−Heating
Application Information
Accuracy depends on the temperature of the remote
sensing transistor and/or the internal temperature sensor
being at the same temperature as that being measured. Many
factors can affect this. Ideally, place the sensor in good
thermal contact with the part of the system being measured.
If it is not, the thermal inertia caused by the sensor’s mass
causes a lag in the response of the sensor to a temperature
change. In the case of the remote sensor, this should not be
a problem since it is either a substrate transistor in the
processor or a small package device, such as the SOT−23,
placed in close proximity to it.
The on−chip sensor, however, is often remote from the
processor and only monitors the general ambient
temperature around the package. How accurately the
temperature of the board and/or the forced airflow reflects
the temperature to be measured dictates the accuracy of the
measurement. Self−heating due to the power dissipated in
the ADT7421 or the remote sensor causes the chip
temperature of the device or remote sensor to rise above
ambient. However, the current forced through the remote
sensor is so small that self−heating is negligible. In the case
of the ADT7421, the worst−case condition occurs when the
device is converting at 36 conversions per second while
sinking the maximum current of 1 mA at the ALERT and
THERM output. In this case, the total power dissipation in
Remote Sensing Transistor
The ADT7421 is designed to work with substrate
transistors built into processors or with discrete transistors.
Substrate transistors are generally PNP types with the
collector connected to the substrate. Discrete types are either
PNP or NPN transistors connected as transistors
(base−shorted to collector). If an NPN transistor is used, the
collector and base are connected to D+ and the emitter to D−.
If a PNP transistor is used, the collector and base are
connected to D− and the emitter to D+. Note that Beta
Cancellation should be turned OFF when using a discrete
transistor. This is done by setting Bit 4 of the Configuration
Register to 1.
To reduce the error due to variations in both substrate and
discrete transistors, consider several factors:
• The ideally factor, n , of the transistor is a measure of
F
the deviation of the thermal transistor from ideal
behavior. The ADT7421 is trimmed for an n value of
1.008. The following equation may be used to calculate
F
the error introduced at a temperature, T (°C), when
using a transistor whose n does not equal 1.008.
F
Consult the processor data sheet for the n values.
F
DT + (nr * 1.008)ń1.008 (273.15 Kelvin ) T)
To factor this in, the user writes the DT value to the offset
register. It is then automatically added to, or subtracted
from, the temperature measurement.
the device is about 4.5 mW. The thermal resistance, D , of
JA
the 8−lead MSOP is approximately 142°C/W.
• Some CPU manufacturers specify the high and low
Layout Considerations
current levels of the substrate transistors. The high
Digital boards can be electrically noisy environments, and
the ADT7421 is measuring very small voltages from the
remote sensor, so care must be taken to minimize noise
induced at the sensor inputs. Take the following precautions:
• Place the ADT7421 as close as possible to the remote
sensing transistor. Provided that, the worst noise sources,
that is, clock generators, data/address buses, and CRT’s
are avoided, this distance can be 4 to 8 inches.
current level of the ADT7421, I
, is 220 mA and the
HIGH
low level current, I , is 13.5 mA. If the ADT7421
LOW
current levels do not match the current levels specified
by the CPU manufacturer, it may become necessary to
remove an offset. The CPU data sheet should advise
whether this offset needs to be removed and how to
calculate it. This offset is programmed to the offset
register. It is important to note that if more than one
offset must be, considered, the algebraic sum of these
offsets must be programmed to the offset register.
• Route the D+ and D− tracks close together, in parallel,
with grounded guard tracks on each side. To minimize
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15
ADT7421
inductance and reduce noise pickup, a 5−mil track
width and spacing is recommended. Provide a ground
plane under the tracks, if possible.
• Place a 0.1 mF bypass capacitor close to the V pin. In
DD
extremely noisy environments, place an input filter
capacitor across D+ and D− close to the ADT7421.
This capacitance can effect the temperature
measurement, so ensure that any capacitance seen at D+
and D− is, at maximum, 2200 pF. This maximum value
includes the filter capacitance, plus any cable or stray
capacitance between the pins and the sensor transistor.
5MIL
GND
5MIL
5MIL
D+
• If the distance to the remote sensor is more than
8 inches, the use of twisted pair cable is recommended.
A total of 6 feet to 12 feet is needed.
5MIL
5MIL
5MIL
5MIL
D–
For really long distances (up to 100 feet), use a shielded
twisted pair, such as the Belden No. 8451 microphone
cable. Connect the twisted pair to D+ and D− and the
shield to GND close to the ADT7421. Leave the remote
end of the shield unconnected to avoid ground loops.
Because the measurement technique uses switched
current sources, excessive cable or filter capacitance can
affect the measurement. When using long cables, the filter
capacitance can be reduced or removed.
GND
Figure 16. Typical Arrangement of Signal Tracks
• Try to minimize the number of copper/solder joints that
can cause thermocouple effects. Where copper/solder
joints are used, make sure that they are in both the D+
and D− path and at the same temperature.
• Thermocouple effects should not be a major problem as
1°C corresponds to about 200 mV, and thermocouple
voltages are about 3 mV/°C of temperature difference.
Unless there are two thermocouples with a big
Application Circuit
Figure 17 shows a typical application circuit for the
ADT7421, using a discrete sensor transistor connected via
a shielded, twisted pair cable. The pull−ups on SCLK,
SDATA, and ALERT are required only if they are not
provided elsewhere in the system.
temperature differential between them, thermocouple
voltages should be much less than 200 mV.
V
3V TO 3.6V
DD
0.1mF
ADT7421
TYP 10kW
CPU
D+
D–
SCLK
5V OR 12V
SMBUS
CONTROLLER
SDATA
SHIELD
ALERT/
THERM2
CPU THERMAL
DIODE
or
2N3906
V
DD
THERM
TYP 10k
W
GND
FAN CONTROL
CIRCUIT
FAN ENABLE
Figure 17. Typical Application Circuit
Name
ADT7421 Register Details
Table 10. Status/Configuration Registers
Read Address
(Hex)
Write Address
(Hex)
Power−On Default
Binary
Hex
Decimal
Not Applicable
Not Applicable
Not Applicable
09
Address Pointer
Undefined
Undefined
02
Status
03
Configuration
Conversion Rate
One−Shot
0000 1000
0000 0110
0x08
0x06
04
0A
Not Applicable
0F
22
3D
FE
FF
22
Consecutive ALERT
Device ID
0000 0001
0010 0001
0100 0001
0x01
0x21
0x41
Not Applicable
Not Applicable
Not Applicable
Manufacturer ID
Die Revision Code
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ADT7421
Table 11. Configuration Register; Read Address 0x03, Write Address 0x09
Bit
Mnemonic
Mask
Read/Write
Description
7
R/W
Setting this Bit to 1 masks all ALERTs on the ALERT pin. Default = 0 = ALERT
enabled. This applies only if Pin 8 is configured as ALERT, otherwise it has no effect.
6
Run/STOP
R/W
Setting this Bit to 1 places the ADT7421 in standby mode (that is, it suspends all
temperature measurements (ADC). The SMBus remains active and values can be
written to and read from the registers. THERM and ALERT are also active in standby
mode. Changes made to the limit registers in standby mode that effect the THERM or
ALERT outputs cause these signals to be updated. Default = 0 = temperature
monitoring enabled.
5
4
ALERT/THERM2
BETA Enable
Reserved
R/W
R/W
This Bit selects the function of Pin 8. Default = 0 = ALERT. Setting this Bit to 1
configures Pin 8 as THERM2 pin.
Setting this Bit to 0 enables Beta Cancellation. Setting it to 1 disables Beta
Cancellation.
3
2
Read only
R/W
Reserved
Temperature
Range Select
Setting this Bit to 1 enables the extended temperature measurement range
(−50°C to +150°C). Default = 0 = (0°C to +127°C).
1
0
Reserved
Reserved
Read only
Read only
Reserved
Reserved
Table 12. Conversion Rate Register (Read Address = 0x04, Write Address = 0x0A)
Bit
Code
Mnemonic
Reserved
Function
Reserved
Reserved
Reserved
Reserved
7
6
5
Reserved
Reserved
4
Reserved
<3:0>
Conversion rates
These bits set how often the ADT7421 measures each temperature
channel.
Conversions/Sec
0000 = 0.0625
0001 = 0.125
0010 = 0.25
0011 = 0.5
Time (seconds)
0x00
0x01
0x02
0x03
0x04
0x05
0x06
0x07
0x08
0x09
0x0A
16
8
4
2
0100 = 1
1
0101 = 2
500 m
250 m
200 m
100 m
50 m
27 m
0110 = 4 = default
0111 = 5
1000 = 10
1001 = 20
1010 = 36
Table 13. Status Register; (Read Address = 0x02)
Bit
7
Name
Function
ALERT
No
BUSY
1 when ADC converting
6
LHIGH (Note 1)
LLOW (Note 1)
RHIGH (Note 1)
RLOW (Note 1)
D OPEN (Note 1)
RTHRM
1 when local high temperature limit tripped
1 when local low temperature limit tripped
1 when Remote 1 high temperature limit tripped
1 when Remote 1 low temperature limit tripped
1 when Remote 1 sensor open circuit
Yes
Yes
Yes
Yes
Yes
No
5
4
3
2
1
1 when Remote 1 THERM limit is tripped
1 when local THERM limit is tripped
0
LTHRM
No
1. These flags stay high until the status register is read, or they are reset by POR
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ADT7421
Table 14. Consecutive ALERT Register (Read Address = 0x22, Write Address = 0x22)
Bit
Name
SCL Timeout
SDA Timeout
Reserved
Function
1 = SCL Timeout enabled. 0 = SCL Timeout disabled = default
1 = SDA Timeout enabled. 0 = SDA Timeout disabled = default
Reserved
7
6
5
4
Reserved
Reserved
<3:0>
Fault Queue
Amount of out−of−limit measurements required for alert
111x = 4
011x = 3
001x = 2
000x = 1
Table 15. Value Registers
Read Address
Write Address
Name
Power−On Default
(Hex)
(Hex)
Binary
Hex
0x00
0x00
0x00
Decimal
00
01
10
Not Applicable
Not Applicable
Not Applicable
Local Temperature Value
0000 0000
0000 0000
0000 0000
External Temperature Value High Byte
External Temperature Value Low Byte
Table 16. Limit Registers
Read Address
(Hex)
Write Address
Name
Power−On Default
(Hex)
Binary
Hex
0x55
0x00
0x55
0x00
0x00
0x00
0x00
0x00
0x55
0x55
0x0A
Decimal
85°C
0°C
05
06
07
08
11
12
13
14
19
20
21
0B
0C
0D
0E
11
Local Temperature High Limit
0101 0101
0000 0000
0101 0101
0000 0000
0000 0000
0000 0000
0000 0000
0000 0000
0101 0101
0101 0101
0000 1010
Local Temperature Low Limit
External Temperature High Limit High Byte
External Temperature Low Limit High Byte
External Temperature Offset High Byte
External Temperature Offset Low Byte
External Temperature High Limit Low Byte
External Temperature Low Limit Low Byte
External THERM Limit
85°C
0°C
12
13
14
19
20
21
85°C
85°C
10°C
Local THERM Limit
THERM Hysteresis
ORDERING INFORMATION
†
Device Order Number*
ADT7421ARZ-REEL
ADT7421ARZ-REEL7
ADT7421ARMZ-REEL
ADT7421ARMZ-RL7
ADT7421ARMZ-2RL
ADT7421ARMZ-2RL7
Package Type
8-Lead SOIC_N
8-Lead SOIC_N
8-Lead MSOP
8-Lead MSOP
8-Lead MSOP
8-Lead MSOP
Part Marking
T7421
T7421
L75
SMBus Address
Shipping
4C
4C
4C
4C
4D
4D
2500 Tape & Reel
1000 Tape & Reel
3000 Tape & Reel
1000 Tape & Reel
3000 Tape & Reel
1000 Tape & Reel
L75
L76
L76
†For information on tape and reel specifications, including part orientation and tape sizes, please refer to our Tape and Reel Packaging
Specifications Brochure, BRD8011/D.
*The “Z’’ suffix indicates Pb−Free package.
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18
ADT7421
PACKAGE DIMENSIONS
SOIC−8 NB
CASE 751−07
ISSUE AJ
NOTES:
1. DIMENSIONING AND TOLERANCING PER
ANSI Y14.5M, 1982.
2. CONTROLLING DIMENSION: MILLIMETER.
3. DIMENSION A AND B DO NOT INCLUDE
MOLD PROTRUSION.
−X−
A
4. MAXIMUM MOLD PROTRUSION 0.15 (0.006)
PER SIDE.
8
5
4
5. DIMENSION D DOES NOT INCLUDE DAMBAR
PROTRUSION. ALLOWABLE DAMBAR
PROTRUSION SHALL BE 0.127 (0.005) TOTAL
IN EXCESS OF THE D DIMENSION AT
MAXIMUM MATERIAL CONDITION.
6. 751−01 THRU 751−06 ARE OBSOLETE. NEW
STANDARD IS 751−07.
S
M
M
B
0.25 (0.010)
Y
1
K
−Y−
MILLIMETERS
DIM MIN MAX
INCHES
G
MIN
MAX
0.197
0.157
0.069
0.020
A
B
C
D
G
H
J
K
M
N
S
4.80
3.80
1.35
0.33
5.00 0.189
4.00 0.150
1.75 0.053
0.51 0.013
C
N X 45
_
SEATING
PLANE
1.27 BSC
0.050 BSC
−Z−
0.10
0.19
0.40
0
0.25 0.004
0.25 0.007
1.27 0.016
0.010
0.010
0.050
8
0.020
0.244
0.10 (0.004)
M
J
H
D
8
0
_
_
_
_
0.25
5.80
0.50 0.010
6.20 0.228
M
S
S
X
0.25 (0.010)
Z
Y
SOLDERING FOOTPRINT*
1.52
0.060
7.0
4.0
0.275
0.155
0.6
0.024
1.270
0.050
mm
inches
ǒ
Ǔ
SCALE 6:1
*For additional information on our Pb−Free strategy and soldering
details, please download the ON Semiconductor Soldering and
Mounting Techniques Reference Manual, SOLDERRM/D.
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19
ADT7421
PACKAGE DIMENSIONS
MSOP8
CASE 846AB−01
ISSUE O
NOTES:
1. DIMENSIONING AND TOLERANCING PER ANSI Y14.5M, 1982.
2. CONTROLLING DIMENSION: MILLIMETER.
D
3. DIMENSION A DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE
BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED
0.15 (0.006) PER SIDE.
4. DIMENSION B DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSION.
INTERLEAD FLASH OR PROTRUSION SHALL NOT EXCEED 0.25 (0.010) PER SIDE.
5. 846A-01 OBSOLETE, NEW STANDARD 846A-02.
H
E
E
MILLIMETERS
INCHES
NOM
−−
0.003
0.013
0.007
0.118
DIM
A
A1
b
c
D
MIN
−−
0.05
0.25
0.13
2.90
2.90
NOM
−−
MAX
MIN
−−
0.002
0.010
0.005
0.114
0.114
MAX
0.043
0.006
0.016
0.009
0.122
0.122
PIN 1 ID
1.10
0.15
0.40
0.23
3.10
3.10
e
0.08
b 8 PL
0.33
M
S
S
0.08 (0.003)
T B
A
0.18
3.00
E
3.00
0.118
e
L
0.65 BSC
0.55
4.90
0.026 BSC
0.021
0.193
0.40
4.75
0.70
5.05
0.016
0.187
0.028
0.199
SEATING
PLANE
H
E
−T−
A
0.038 (0.0015)
L
A1
c
SOLDERING FOOTPRINT*
1.04
0.38
8X
8X 0.041
0.015
3.20
4.24
5.28
0.126
0.167 0.208
0.65
6X0.0256
SCALE 8:1
mm
inches
ǒ
Ǔ
*For additional information on our Pb−Free strategy and soldering
details, please download the ON Semiconductor Soldering and
Mounting Techniques Reference Manual, SOLDERRM/D.
Protected by US Patents 5,195,827; 5,867,012; 5,982,221; 6,097,239; 6,133,753; 6,169,442; 7,010,440; other patents pending.
ON Semiconductor and
are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice
to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any liability
arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages.
“Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All
operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. SCILLC does not convey any license under its patent rights
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